Today, the understanding of environmental concerns is of great significance, making it desirable for children to investigate the scientific concepts underlying a growing list of emerging environmental issues. This paper deals with young children's preconceptions. The authors have stressed the importance of investigating children's ideas at an early stage, with some of these ideas leading to knowledge far removed from scientific knowledge. Our study focused on the knowledge about organic matter processes among five-year-old children who were only exposed to informal learning within the context of home waste management. We used semi-structured interviews with 48 young children to determine their knowledge about the organic matter cycling in composters. The determination of five significant stages for a global understanding of the cycle of organic matter and the construction of three levels of formulation have allowed us to conclude that young children are able to understand scientific processes involved in composting in the context of their family practice of organic waste management.